The present invention relates to electronic imaging apparatus and methods. More particularly, it concerns apparatus and methods for forming optical images of a scene on electronic devices that convert the optical image irradiance falling on them to an electronic output signal that faithfully represents the scene with a minimum of ambiguity in the final reconstituted image.
In electronic imaging systems, radiation reflected from a scene is imaged on the photosensing surface of the electronic imaging device. In television imaging systems, an optical system directs scene radiation into a vidicon tube to present the scene image at a photoconductive target surface located at an image plane. The target surface is electronically charged to a local charge level by the imaged irradiance pattern of the scene which is proportional to the corresponding local radiance of the scene. The target is scanned in a raster pattern by an electron beam to provide an output signal which electrically represents the target surface charge pattern caused by the imaged scene irradiance. In this way, a continuous real time electrical replica of the scene radiation is generated for subsequent use.
Semiconductor imaging devices suitable for use in hand-held still cameras include various types of monolithic and hybrid area imaging photoresponsive sensors such as charged coupled devices (CCDs) and photodiode arrays. In a CCD area imaging sensor, a two dimensional metal insulator semiconductor (MIS) substrate is formed from a semiconductor material with a series of electrodes formed on one surface. The opposite image receiving surface is exposed to scene irradiance to create and store packets of photogenerated charges in the substrate. After a suitable sampling interval, the charge packets are transferred in a sequential step-wise manner to an output device to provide an electrical signal representative of the sampled scene. In addition to area imaging devices, linear arrays, or even a single detector, can be used with devices which scan the scene across the array to provide a corresponding video signal representative of the scene.
In the fabrication of semiconductor image sensing arrays, the number of photosensitive areas or "pixels" which can be provided per unit area is limited by practical material and fabrication considerations. Because the number of pixels is limited, the ability of such devices to resolve spatial detail in an image is correspondingly limited. The image producing quality of an imaging device is oftentimes expressed in terms of its modulation transfer function (MTF), the ratio of modulation in the image to that in the object. Generally, the higher the value of the MTF with spatial frequency, the better it will resolve spatial detail in an object. All such devices, however, have a practical spatial frequency limit at which their modulation transfer function value drops to an unacceptably low value. To put it in other terms, the electrical output signal of an imaging device can be expressed as a Fourier transform. Ideally, the electrical transform provided by the imaging device is an exact analog of the corresponding transform which describes the two dimensional flux density distribution at the image plane as provided from the scene. When an electrical imaging device that periodically samples a scene containing spatial frequency components much higher than the Nyquist frequency of the device, the frequency components above the Nyquist frequency limit of the device will in reconstruction appear as spurious lower frequency components. These spurious components are known as "aliases" and are defects in the electrical output signal. While aliasing can be controlled by providing devices having higher sampling rate, the aliasing effect limits the ultimate picture taking ability of an electronic imaging camera.
A principal objective of the present invention is, therefore, the provision of an improved electronic imaging system for providing an electrical signal which faithfully represents a scene with a minimum of imperfections caused by high frequency components in the scene radiation. Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings, in which like parts are designated by like reference characters.